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Limbert recommended that due to unexplored nature of cave, the trial tours, to be held twice a month, should consist of a group of only seven or eight visitors.

Son Doong Cave, found in Phong Nha-Ke Bang National Park in 1991, is considered the largest cave in the world. It contains a large fast-flowing underground river inside its walls.

A group of British scientists from the British Cave Research Association, led by Limbert, conducted a survey in 2009.

Limbert suggested that in addition to ensuring the safety of the tourists, authorities and travel companies should protect the primary site and the surrounding ecological environment.

Exploration of the cave is preferential in dry season, between February and August.

The trial tours will be launched next year before operating officially.

VNS/VOV online